The present invention relates to a crane or derrick, and, more particularly, to a crane or derrick for installation and use on an offshore platform.
In conducting drilling operations from offshore platforms, as is well known to those skilled in the art, it is usually necessary to utilize a derrick barge to load heavy equipment on to the platform deck, the derrick barge usually being provided with a crane or similar lifting apparatus for lifting such equipment. Additionally, as pointed out in U.S. Pat. No. 4,144,940, it is generally necessary to use a derrick barge for driving conductor pipe and piling, preliminary to actual drilling operations, as the offshore platform is not normally provided either with equipment to actually drive pile or conductor pipe, or to install apparatus on the platform, e.g. a pile driving rig, for carrying out those operations. The use of a derrick barge is expensive, particularly if the barge has to remain at the offshore platform location for any period of time. This is generally the case with a new offshore platform which usually is not provided with a crane or other lifting device suitable for raising heavy loads onto the deck of the platform. Moreover, stationing of a derrick barge adjacent an offshore platform can pose a severe safety hazard should turbulent weather be encountered. It would clearly be more economical and safer to have a lifting means on the platform deck which could lift equipment from a less costly supply barge or supply boat to the deck of the platform. It is known to erect lifting devices such as stiff leg cranes on the deck of the platform in order to provide a lifting means to lift equipment and the like from supply boats onto the platform deck. However, such stiff leg cranes, because of their size, do not have the lifting capacity to raise heavy loads to the platform deck.
It will be recognized that in a crane or derrick of the fixed guy or stiff leg type, the higher the kingpole or mast, the larger the lifting capacity of the crane or derrick because of the higher center of gravity However, a kingpole or mast which is tall enough to permit the lifting of heavy loads must also possess sufficient structural integrity so as to resist the compressive and tensile forces acting on the kingpole when the crane is in use. This means that the kingpole is intrinsically heavy as it will generally be made of thick walled pipe or similar, heavy tubular material. Thus, while it would be desirable to have a crane on the platform deck which could lift heavy enough loads to eliminate the need for a derrick barge, the size of such a crane virtually dictates that a derrick barge must be employed to erect such a crane on the platform deck.